Such heaters have been known as auxiliary heaters for heating passenger compartments of vehicles, boats or construction equipment while the engine of the vehicle is not in operation, and have been commercially available for many years. These prior-art heaters are designed for a defined rated capacity, so that sufficient heating with the vehicle engine not running or additional heating with the engine running and consequently supplying heat is possible even under extreme conditions. If only a small amount of heat is to be supplied, this is achieved in the prior-art heaters either by intermittent operation or by operation at partial load, usually at 1/4 load at the lowest level.
It has been known that the burners of such heaters are operated, regardless of the mode of fuel supply and the mixture preparation, with an air ratio of lambda .lambda.=1.2-1.5 at full load. To operate the burner in the partial load range, the amount of fuel must be reduced corresponding to the changed output ratio (partial load:full load). To achieve satisfactorily clean combustion, the amount of combustion air must be adjusted as well. Investigations carried out in this connection have shown that air ratios of lambda .lambda.=2 lead to the relatively best combustion values at the 1/4 partial load that is commonly used currently. However, this depends essentially on the design of the burner. Thus, air ratios in the range of lambda .lambda.=3-4 are obtained for some prior-art combustion chambers. These high air ratios are necessary in order to achieve still satisfactorily clean combustion with still tolerable emission values in the partial load range. The necessary reduction of the amount of combustion air is brought about in the prior-art heaters corresponding to the partial load-to-full load ratio by reducing the fan speed, which does not, of course, take place in direct proportion to the reduction of the fuel supply for the reasons shown.
However, the requirement to operate a heater with a high air ratio in the partial load range has the disadvantage that the excessive supply of combustion air leads to cooling of the flame and it consequently adversely affects burn-out. Another disadvantage is that the heater cannot be operated at partial loads lower than ca. 1/4, even though this is no longer sufficient for satisfying the increased demands on comfort. Ranges of 1:10 to 1:15 are now desirable especially in utility vehicles. For example, preheating the engine of a truck requires a heating output of 10 kW, whereas 1 kW is sufficient for heating the sleeping box.
An ultrasonic atomization burner of this class for air heaters of lower output for vehicles, in which heater combustion air is fed into the combustion space through air supply openings via an annular space surrounding the combustion chamber, has been known from DE 33,18,054 A1. Regardless of the type of fuel preparation (evaporative burners, mechanical atomizers, ultrasonic atomizers, etc.), it is not possible to operate these devices in partial load ranges below 1:4 with still acceptable combustion properties.